Third-generation mobile communication systems for mobile phones, etc., such as cdma 2000 and W-CDMA using a spread spectrum (Code Division Multiple Access: hereinafter referred to as “CDMA”) scheme, have been used. For a W-CDMA downlink, a 3.5th-generation packet transmission scheme called HSDPA (High-Speed Downlink Packet Access) is known as a packet data transmission technique for providing a transmission rate of up to about 14 Mbps.
HSDPA employs an adaptive coding modulation scheme (AMC: Adaptive Modulation and Coding) that allows transmission at a power receivable by all terminal devices located within an area covered by a base station and that allows an optimum modulation scheme to be used depending on the radio wave conditions (CQI information) between the base station and the terminal device, thereby achieving high-quality and high-speed communication. There are available, as modulation schemes, a QPSK modulation scheme with high stability like W-CDMA, which is one type of PSK (Phase Shift Keying) modulation scheme, and a 16-QAM scheme allowing high-speed communication, which is one type of QAM (Quadrature Amplitude Modulation) modulation scheme. Those two modulation schemes are currently used. However, the PSK schemes further include BPSK and 8PSK in addition to QPSK, and the QAM schemes further include 64-QAM and 256-QAM in addition to 16-QAM. That is, there are modulation schemes with different modulation levels.
The above-mentioned CQI (Channel Quality Indicator) information is a result of measurement of the signal quality (for example, SIR) by the terminal device from CPICH (common pilot channel) received from the base station. The CQI information is transmitted to the base station via HS-DPCCH (High Speed Dedicated Physical Control Channel.
Primary radio channels for use in HSDPA include HS-PDSCH (High Speed-Physical Downlink Shared Channel) and HS-SCCH (High Speed-Shared Control Channel) as shared channels in the downlink direction (from the base station to the communication terminals). HS-SCCH is a control channel carrying various control information, such as address information and a modulation scheme of packet data to be transmitted via HS-PDSCH, and error correction parameters. The terminal device receives HS-SCCH, and can recognize the modulation scheme used for the HS-PDSCH, the error correction parameters, etc., to perform demodulation and decoding for the HS-PDSCH.
Further, HSDPA employs a method in which one physical channel is shared in time division and is used by a plurality of communication terminals. A scheduler located in the base station performs scheduling to determine a communication terminal to which packet data is to be transmitted and transmission parameters of the packet data in predetermined units of time according to the amount of data to be transmitted to the communication terminal, the communication quality, and the priority, thereby achieving efficient packet data transmission.
Several algorithms for the scheduler to determine the transmission parameters have been proposed. A Max CIR (Maximum Carrier to Interface power Ratio) method, an RR method (Round Robin) method, and a PF (Proportional Fairness) method that allow efficient scheduling are known. For example see Yoshiaki Oofuji, et. al., “Kudari Rink Kosoku Paketto Akusesu ni okeru kaku Yuza no Suruputto ni Chumoku shita Sukejuringuhou no Tokusei Hikaku (Characteristic Comparison between Scheduling methods focusing on Throughput of Each User in High-Speed Downlink Packet Access),” Technical Report of IEICE SST2001-108. A•P2001-256. RCD2001-291, MoMuC2001-88, MW2001-226 (2002-03).
In the Max CIR method, a user having the highest transmission rate is preferentially assigned a slot on the basis of the reported SIRs at the terminals. In the RR method, all users are equally assigned slots regardless of the SIR. In the PF method, a terminal device having the highest ratio of an average reception SIR value for the terminal device to the instantaneous SIR of each user is assigned a slot. Which algorithm is to be selected depends on what evaluation amount is to be prioritized for a system.
In order to realize a mobile communication scheme that allows higher-speed communication, studies on fourth-generation communication schemes have been extensively carried out. Further, in 3GPP (3rd Generation Partnership Project), studies on an LTE (Long Term Evolution) scheme, which is considered as the 3.9th generation, have been made. As an LTE communication scheme, orthogonal frequency division multiplexing (hereinafter referred to as “OFDM”) is used, instead of the CDMA scheme, for realizing high-speed communication (for example, see 3GPP TR25.913 standard).
The OFDM scheme employs a method in which a carrier for modulating data is divided into a plurality of sub-carriers orthogonal to each other and data signals distributed to the sub-carriers are transmitted in parallel. By performing transmission while maintaining the straight-traveling characteristics between the plurality of sub-carriers, optimum transmission efficiency for high-speed data communication can be attained.
In mobile communication based on the OFDM scheme, unlike the CDMA scheme, no spread code is used, and there is a problem in that adjacent base stations may interfere with each other when using the same channel at the same time. Therefore, each base station needs to use a different channel. However, if available channels are limited, the number of users that can be accommodated is also limited. Thus, because of the limited number of available channels, it is desirable that adjacent base stations can use the same channel.
In order to solve this problem, a method in which a frequency reuse distance is variably set according to the distance from a terminal device to a base station has been proposed (see for example 3GPP TSG-RAN WG1 Tdoc R1-051341). For example, as shown in FIG. 1, in a case where the total number of channels (a plurality of sub-carriers orthogonal to each other) available for communication is three, namely, CH1, CH2, and CH3, a terminal device (for example MS) that is distant from the base station (in a cell boundary area 101) is permitted to use only the CH1 by increasing the reuse distance of CH1, and is not permitted to use the same frequency as an adjacent base station, thereby minimizing any interference on neighboring cells. A terminal device that is close to the base station (in a cell center area 102) is permitted to use all the frequency channels CH1, CH2, and CH3, thus reducing the reuse distance, thereby increasing the channel use efficiency.
However, in considering a case where the conventional HSDPA scheme is directly used for OFDM communication in the downlink direction, there is a problem in that since a base station performs transmission to terminal devices at a power that allows transmission signals to reach terminal devices located in a distant area from the base station (the cell boundary area 101 shown in FIG. 1), the above-mentioned method in which the frequency reuse distance is variably set cannot be used for communication in the downlink direction, thus causing interference in a communication area of an adjacent base station.
In a case where HSUPA (High Speed Uplink Packet Access) for communication in the uplink direction, which corresponds to HSDPA, is used in the OFDM scheme, there is a problem in that since a terminal device performs transmission at a power that allows a transmission signal to reach a base station even if the terminal device is located anywhere in the communication area, interference may occur in a communication area of an adjacent base station.
The above-mentioned problems with a conventional mobile communication system that uses the HSDPA and HSUPA schemes for the OFDM scheme will be described with reference to FIGS. 2 and 3. It is assumed that the mobile communication system is composed of base stations BS1 and BS2 and terminal devices MS1 and MS2, and that the total number of channels available is three, namely, CH1, CH2, and CH3.
A cell 100-1 that is a communication area of the base station BS1 can be divided into a cell boundary area 101-1 and a cell center area 102-1. In the cell boundary area 101-1, an adjacent frequency reuse distance is increased, and only CH1, which is a channel (hereinafter referred to as a “first channel”) assigned so as not to interfere with an adjacent base station, is used. In the cell center area 102-1, the frequency reuse distance is reduced, and all the channels CH1, CH2, and CH3 are used. The channels (CH2 and CH3) other than the first channel are hereinafter referred to as “second channels.”
A cell 100-2 that is a communication area of the base station BS2 can also be divided into a cell boundary area 101-2 and a cell center area 102-2. In the cell boundary area 101-2, only the CH1, which is a channel assigned as the first channel, is used. In the cell center area 102-2, all of the first channel CH1 and the second channels CH2 and CH3 can be used.
Under this constraint, the terminal device MS1 is located in the cell center portion 102-1, and can use all the channels CH1, CH2, and CH3. The terminal device MS2 is located in the cell boundary area 101-2, and can use only CH1.
As shown in FIG. 2, if the terminal device MS1 uses the CH2, TPC (transmission power control) is used for transmission via the individual channels to reduce the transmission power from the base station BS1 to the terminal device MS1, resulting in no interference with the adjacent cell 100-2. However, in a case where a transmission method using a shared channel, such as HSDPA, is used, the TPC control is not carried out, and transmission is performed at a power that covers the entire cell. Thus, the base station BS1 performs transmission to the terminal device MS1 located in the cell center area 102-1 at the same power as that for a terminal device located in the cell boundary area 101-1. Therefore, communication in the downlink direction using the CH2 from the base station BS1 causes interference with the terminal device MS2.
If both the terminal devices MS1 and MS2 share the CH2 in the HSUPA scheme, as shown in FIG. 3, the MS2 performs transmission at a power sufficient to reach the base station BS2 from the cell boundary area, however communication in the uplink direction from the terminal device MS2 causes interference with the base station BS1.